Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
An ion implanter usually includes an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam may be mass analyzed to eliminate undesired ion species, accelerated to a desired energy, and directed onto a target plane. The ion beam may be distributed over the target area by beam scanning, by target movement, or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam, the latter of which includes a long dimension and a short dimension. The long dimension may be at least as wide as the wafer.
Introducing the impurities at a desired dose into a wafer is important to ensure that the semiconductor device being formed operates within specification. One factor that can affect the impurity dose into the wafer is the ion beam current distribution. Beam uniformity is required to provide a precise, uniform impurity dose. An unexpected fluctuation in ion beam current may degrade the uniformity of the impurity dose. If the ion beam is not uniform, it can lead to different concentrations of ions penetrating a wafer in different regions.
With advances in semiconductor manufacturing, there is an increased need for process information to, for example, provide process assurance or device modeling data. Thus, more accurate and complete measurements of beam properties, such as beam uniformity, may be required. More accurate and complete beam property measurements may also reduce process risk during implant through detection of a non-uniform beam.
When measuring ion beam uniformity at a user-specified interval, known as a “Uniformity Check Interval,” a user often finds that uniformity is not within specification. Such intervals may be, for example, every hour or every 100 wafers. A “Uniformity Check Interval” does not provide feedback during implantation of a wafer.
Some single wafer implanters assume that if the beam was uniform during setup then uniformity is unchanged during wafer processing unless a uniform change in beam current across the entire beam occurs. Some scanned beam machines, ribbon beam machines, and spot beam single wafer machines may utilize this method because they rely on beam shape being unchanged to meet their uniformity specification at high throughput. This method also does not provide feedback on beam uniformity during implantation of a wafer.
Some single wafer implanters rely on current measurement of the beam across the entire wafer to test for beam uniformity during setup. Single point measurements or single numbers are typically relied upon during ion implantation to represent the beam. This generally only provides information as to whether the beam current has gone up or down during setup. Single point measurements do not check whether beam uniformity has been maintained.
Accordingly, there is a need in the art for new and improved methods and apparatus for improved monitoring of beam uniformity.